Rare earth magnets are used in a wide variety of applications including household electric appliances, large computer peripheral terminals, and medical equipment. They constitute a family of very important electronic materials, which is a key to the advanced technology. In compliance with the recent trend of reducing the size and weight of computers and communication equipment, efforts have also been made to develop rare earth magnets of reduced size and increased precision. Since rare earth magnets are expected to find further spreading applications, there will be a rapid increase of the demand for rare earth magnets.
Rare earth magnets are generally molded to a rough estimate size, sintered, machined and ground to the predetermined size, and surface finished into commercial articles as by plating or coating. These operations generate scraps including powder surplus upon molding, failed or defective articles associated with sintering, machining and plating, and under-performing articles, which amount to ten or more percents of the initial weight of raw material. The machining and grinding operations generate sludge or swarf including machined chips, waste and dust, which also amount to several ten percents of the initial weight of raw material. From the standpoints of saving the resource, reducing industrial wastes, and reducing the cost of rare earth magnets, it is very important to recover rare earth elements from the rare earth magnet scrap and sludge for reuse.
In the rare earth magnet manufacturing process, it is almost unavoidable that gaseous impurities such as oxygen and carbon are introduced into rare earth magnets. The sludge includes fine particles of the magnet composition and rare earth oxides, which are likely to oxidize due to their high activity. Since an organic solvent included in a coolant fluid used in the machining operation sticks to sludge surfaces, the sludge has a concentration of carbon, nitrogen and hydrogen, which is several hundred to thousand times higher than that of normal alloy powder.
In general, rare earth has a very high affinity to gaseous components such as oxygen and carbon, which impedes the removal of such gaseous components. It is thus deemed very difficult to recover rare earth elements from the rare earth magnet scrap and sludge for reuse.
Heretofore, several methods have been proposed for the reclamation of rare earth magnet scrap or sludge. Depending on the reclamation or reuse form of rare earth element, these methods are divided into three classes, (1) rare earth recovery, (2) alloy reclamation and (3) magnet reclamation.
The rare earth recovery method is to recover only rare earth elements from magnet scrap or sludge as rare earth compounds, which are recycled to the raw material stage. More specifically, the scrap is dissolved using an acid, after which the solution is chemically treated to recover rare earth elements as fluorides or oxides. This is followed by calcium reduction or molten salt electrolysis, thereby obtaining rare earth metals. For example, Japanese Patent No. 2,765,740 discloses a method for separating and recovering rare earth elements by dissolving rare earth magnet scrap in an aqueous nitric acid/sulfuric acid solution, and adding an alcohol to the solution whereupon crystallized rare earth sulfate is selectively precipitated out. JP-A 9-217132 discloses a method for separating and recovering rare earth compounds and cobalt by adding nitric acid to a slurry of a cobalt-containing rare earth-iron base alloy, and adding oxalic acid or fluoride to the solution containing cobalt and rare earth elements. These methods have the advantages that a large quantity of scrap or sludge can be treated at a time and rare earth compounds of high purity can be recovered, but suffer from several problems including use of a large volume of acid, difficult disposal of used acid and complex steps.
The alloy reclamation method is characterized in that the magnet scrap or sludge is recovered as an alloy of the same composition. The scrap is melted by high frequency melting, arc melting or plasma melting, obtaining a magnet alloy. For example, in JP-A 8-31624, rare earth magnet scrap is melted together with a magnet raw material by high-frequency melting whereby the scrap is reclaimed as a magnet alloy. JP-A 6-136461 utilizes a zone melting technique to separate a rare earth magnet scrap into an alloy and slag. These methods have the advantages that by reclaiming the scrap as a magnet alloy, the smelting step of obtaining a rare earth-containing alloy and the melting step of obtaining a magnet alloy are shortened, and the expensive transition metals which are included in the magnet scrap along with the rare earth elements can also be recovered. Undesirably, the percent recovery of rare earth elements is low, and the crucible material can be eroded away and introduced into the ingot as foreign matter.
In contrast, the magnet reclamation method is to reclaim the scrap or sludge as a magnet. For example, Japanese Patent No. 2,746,818 discloses a method of obtaining a magnet by grinding magnet scrap, admixing it with a predetermined proportion of a rare earth-rich alloy powder, compacting the mixture and sintering. In the method of this patent, solid scrap and rare earth alloy are loaded together in a crucible, before they are heated and melted in a high-frequency melting furnace whereby a magnet-forming alloy is reclaimed. This method is economically advantageous because the existing magnet manufacturing apparatus can be utilized and not only rare earth elements, but also expensive transition metals can be recovered and recycled. It is also contemplated that about 10% by weight based on the melting feeds of a rare earth alloy is melted together in order to prevent erosion of the crucible material, and a flux is added in order to reduce the amount of slag generated which is believed to cause erosion of the crucible material.
However, since the scrap accounts for 90% of the melting feeds, the percent yield of this method is very low when no flux is added. This requires that the flux be added in an amount as large as 40% of the melting feeds. The flux causes the crucible to be eroded so that the crucible material is introduced into the ingot, exacerbating the magnetic properties and surface treatment amenability of the alloy ingot. There also arise issues including a reduced recovery rate of rare earth and an increased cost of operation.
During the magnet manufacturing process, 0.05 to 0.8% by weight of oxygen is inevitably introduced in the solid scrap. If the solid scrap alone is remelted in a high-frequency-melting furnace, the rare earth elements instantaneously form oxides to reduce the recovery rate of rare earth from the solid scrap. Furthermore, the rare earth oxides thus formed are dispersed throughout the molten metal and interconnected in a network form so that the molten metal resides in the network of oxide, resulting in poor separation between the molten metal and the slag and a reduced recovery rate of the ingot.
It is noted that when feed materials of low quality such as scrap are melted, more slag generates. Due to the very poor separation of the thus generated slag and the molten metal, a substantial portion of sound molten metal is entrained in the slag and left in the crucible, leading to a lowering of the recovery rate of the ingot. To solve the above problems, several methods have been proposed.
One exemplary method of preparation of a high purity rare earth metal is a defluorinating method involving heating and melting a rare earth metal and a fluoride thereof together, removing oxygen therefrom, and remelting in high vacuum. Since a large amount of fluoride is added, a crucible made of tantalum or analogous metal must be used in order to avoid erosion of the crucible. To remove the fluorine introduced as the impurity, remelting operation is necessary.
In the above-described method of preparing a magnet-forming alloy by charging a crucible with magnet scrap together with a rare earth alloy, heating and melting them, and then adding a flux and scrap, the amount of the flux added is as large as 40% of the melting feeds, with a likelihood that unmelted flux can be left and carried into the ingot. Further, during vacuum pumping and flux addition, the flux will scatter and be incorporated into the ingot, exacerbating the magnetic properties and surface treatment amenability of the resulting magnet.